A new class of rechargeable batteries based on lithium ion or rocking chair technologies is entering the marketplace in such consumer applications as power sources for camcorders, cellular telephones, portable computers, and the like. Cells making up these batteries offer advantages over competing systems in terms of high operating voltages (&gt;3 V), energy density, and cycle life. However, as with all stored energy devices, there are risks associated with abuse. In such lithium ion cells, a problem of ignition exists if the cell is overcharged to too great an extent beyond the recommended charging voltage. Under such circumstances, the cell can heat up, vent, and ignite unless certain safety devices are incorporated. Lithium rechargeable cells are unsafe under certain recharge conditions. Charger control is effective at providing protection, but cells can contain internal pressure activated disconnect devices as additional protection. A reliable means of activating the disconnect at a specified point during overcharge is in demand. Generated gases can be used and have been described elsewhere.
U.S. Pat. No. 4,943,497, Oishi et al., Sony Corporation, describes the problem associated with overcharging in such cells and presents a method of protection. Incorporation of an internal switch or disconnect device is used to open circuit the cell at an appropriate point during overcharge abuse. An increase in internal cell pressure is used to activate the device. Gases generated as a result of electrolyte decomposition at the cathode at elevated voltages is used as the means of pressure generation.
Oishi et al. disclose a cell having an explosion proof valve which is deformable upon increase of inner pressure of the cell to cut a connection lead which connects the valve with a generator unit contained in the cell. The supply of charging current is cut off when the inner pressure of the cell has abnormally increased. The gradual increase of the inner pressure can be effected by selecting suitable cathode active material.
The cell design disclosed in U.S. Pat. No. 4,943,497 works reasonably well in that a useful commercial product can be made that is adequately protected internally on overcharge. Gas generation mechanisms that are relied upon to activate the disconnect device are functions of cell voltage, temperature, and time. The rate of gas generation is not constant, but increases with cell voltage and temperature. Quite importantly to actual cell applications, gas generation continues with time at a given voltage and temperature. Thus, while gas generation is required to ensure safe shutdown of the cell during overcharge, prolonged gas generation must be avoided during normal operation. Otherwise the disconnect device can become triggered during normal usage as gaseous decomposition products accumulate with time, and gas pressure rises.
In practice, both requirements can be met with judicious choice of electrolytes and cathode materials. However, these restrictions prevent certain other combinations from being chosen that are advantageous for other reasons (such as cost, complexity, energy density, etc.) Also, a maximum lifetime to the disconnect functioning is imposed if gas generation is continuous in an otherwise perfectly sealed cell.
Oishi et al. disclose a cell that achieves safe shutdown on overcharge by using a combination of cathode active materials to ensure sufficient gassing occurs on overcharge. In the embodiment described, cells made with 100% LiCoO.sub.2 cathode only experienced 90% failure on overcharge. Thus, under the conditions described, a satisfactory cell could not be made using 100% LiCoO.sub.2 cathode.
U.S. patent application Ser. No. 927,824, now abandoned, A. Rivers-Bowerman et al. (Moli), discloses an improved cell disconnect device. The electrochemical cell has a current cutoff means for preventing current flow. The current cutoff means has a burstable vent for preventing a dangerous explosion from occurring within the cell. The current cutoff means operates upon the exertion of a pressure in excess of a first predetermined pressure. The burstable vent operates upon the application of pressure exerted directly on the diaphragm in excess of a second predetermined pressure exceeding the first predetermined pressure.
Japanese Laid Open Pat. App. No. 294373/89 (Sony Corporation) discloses void requirements for a cell with a disconnect device. The battery is constructed of a container which accommodates (a) a negative electrode consisting of a calcinated organic body, (b) a positive electrode containing Li.sub.x MO.sub.2 (where M represents at least one kind of either Co or Ni, and x is equal to or greater than 0.05 and equal to or less than 1.10), and (c) an electrolytic solution. The battery provides a cavity with a volume of 0.3 cc or greater per capacity of 1 AH.